The present invention is related to the alignment and registration of components onto substrates in a machine placement environment. More particularly, the present invention is directed a process for in situ reflow of a flip chip-type semiconductor product in a manner which promotes proper settling of the chip onto a substrate as solder begins to flow. The present invention may also be used with any type of component to substrate soldering or bonding where it is useful to apply a downforce to assist in holding alignment prior to a change taking place in the bonding material, the change affecting the applied downforce.
Robotic assembly equipment is well known in the art. Such equipment includes, for example, pick and place (or placement) machines. A placement machine is a robotic instrument for picking up electronic and similar parts from component feeders and placing them at their assigned locations on a substrate such as a printed circuit board (PCB). Once all parts are placed, the PCB is placed in a reflow oven and solder paste disposed on the PCB melts or xe2x80x9creflowsxe2x80x9d forming permanent electrical connections between conductive pads on the PCB and electrical contacts, leads or xe2x80x9cpinsxe2x80x9d on the electrical components.
Occasionally there are problems with the permanent electrical connections. For example, two pads of the PCB may become inadvertently bridged by solder, forming a short; the component may be mis-located; the component may prove faulty; and the like. In these situations, it is often economically desirable to salvage the partially assembled PCB rather than to scrap it. In order to salvage the PCB, one must remove the faulty component, re-prepare the PCB surface, and place and solder a new component (or a cleaned component) in the correct position on the PCB. This process is termed xe2x80x9creworkxe2x80x9d. Reworking thus involves reflowing the solder of an identified target component (and not that of the entire PCB), removing the faulty component; cleaning and refluxing the PCB in the location where the component is to be mounted, reinstalling the component and reflowing the solder for the component.
In the past, most known placement systems locate the part over the substrate, place it, and then the part is released, placed in a reflow oven, and allowed to reflow. Generally the surface tension properties of the molten solder cause the pins of the part to more or less self-center on corresponding pads of the substrate resulting in a good electrical contact. Similarly, known rework systems rely on the self-centering on the pins of the part to the pads of the substrate to achieve accurate placement. While the existing systems operate relatively effectively, as pin densities increase, it is becoming more desirable to exert additional control on the placement of flip chip-type parts, particularly as the value of such parts tends to be higher than other electronic parts used in the fabrication of PCBs.
A method and apparatus for component to substrate assembly permits in situ reflow of a flip chip (or other suitable component) in a manner which promotes proper settling of the component as solder begins to flow at the contact points between the component and the substrate. The pick-up head of a placement machine heats the component while applying up to several grams of downward force that serves to level the component. The downward force (downforce) is accurately measured using an electronic force sensor such as a strain gauge, force sensitive resistor, or any other suitable type of force sensor. The initiation of solder reflow can be detected with the pick-up head by sensing a decrease in the downforce. At this instant, the downforce applied to the component with the pick-up head is decreased preferably to zero and the vacuum or other retention mechanism holding the component is then released, freeing the component from the pick-up head and permitting the component to properly self-center using the liquid solder""s surface tension. Further, at the instant that solder reflow is detected, the pick-up head may optionally be displaced a short distance from the component. However, because the pick-up head must (where it is used to supply heat) continue to supply heat to complete the reflow of the solder, it is only displaced a minimal distance from the component so that heating by radiation continues to reflow the solder while the pick-up head is displaced from the chip. The approach is applicable to other assembly processes where downforce is helpful to stabilize a component prior to final bonding and a change in measured downforce indicated the beginning of melting, curing or another process which indicates that downforce can be removed.